Reliability and sensitivity analyses of discrete fracture network based contaminant transport model in fractured rocks

Abstract

Understanding the flow and transport of contaminants in fractured rocks play a fundamental role in geo-environmental problems since subsurface contamination poses a serious threat to human health and the environment. Therefore, it is imperative to develop predictive models and understand the potential impact of contaminant movement in the subsurface environment (i.e., fractured rocks). In this paper, a two-dimensional numerical model is developed to study the groundwater flow and contaminant transport behavior in fractured rock mass based on the discrete fracture network approach. The effect of aperture variation along the fractures is also incorporated as an additional feature in the model. The contaminant transport through heterogeneous fracture network and the intact rock matrix is simulated, the contaminant concentration evolving over spatial and temporal scales is computed, and the influence of various transport mechanisms on the contaminant migration process is also discussed. Uncertainty analysis forms an intrinsic part of predictive modelling. Hence, the stochasticity in the fracture pattern generation, uncertainties in the geological properties of the medium, transport properties of the contaminant are considered, and efficient probabilistic techniques are employed to quantify the effect of these uncertainties. The probability of contaminant concentration exceeding the permissible limit (known as probability of failure) is estimated by implementing computationally effective subset simulation method. The influence of the coefficient of variation in the uncertain input parameters of the model, and the stochasticity in the fracture network on the probability of failure are also studied. Further, the critical parameters affecting the concentration of the contaminant are estimated by performing sensitivity analysis. Thus, the probabilistic framework presented in this study highlights the impact of various uncertainties on the contaminant migration process. It also provides a technical basis for the engineering practitioners in identifying the key areas that require utmost attention while planning contaminant site remediation or designing waste disposal system.